It should also be noted that the transition from KNOX
expressing to KNOX nonexpressing tissue at the SAM base
is not as sharp as the transition shown by other molecular
markers. Most notably, markers for photosynthesis (such as
chlorophyll and RBCS) are totally absent from the SAM but
are apparent within a few cell diameters in the subtending tissue
(Fleming et al., 1996). Because exposure of the SAM to light
does not lead to the SAM cells expressing these markers, there
is a developmental control excluding or preventing SAM cells
undergoing this pathway of differentiation. The nature of this
control is essentially unknown, yet it represents one of the most
basic questions in cell biology, i.e. what controls the switch
of a meristem cell to a particular differentiated pathway?
Transcriptional regulators, such as the KNOX gene products,
clearly play a role in setting the window within which such events
can occur, but the actual molecular mechanism by which the
switch occurs remains unknown.